Light emitting device

ABSTRACT

A light emitting device includes: a first layer made of a semiconductor of a first conductivity type; a second layer made of a semiconductor of a second conductivity type; an active layer including a multiple quantum well provided between the first layer and the second layer, impurity concentration of the first conductivity type in each barrier layer of the multiple quantum well having a generally flat distribution or increasing toward the second layer, average of the impurity concentration in the barrier layer on the second layer side as viewed from each well layer of the multiple quantum well being equal to or greater than average of the impurity concentration in the barrier layer on the first layer side, and average of the impurity concentration in the barrier layer nearest to the second layer being higher than average of the impurity concentration in the barrier layer nearest to the first layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2008-203039, filed on Aug. 6,2008; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a light emitting device.

2. Background Art

A white LED with high emission efficiency realizes an illuminationdevice that can replace incandescent lamps and fluorescent lamps andfacilitates low power consumption. Furthermore, a blue-violetsemiconductor laser device with high emission efficiency facilitatesenhancing the performance of DVD (digital versatile disc) and otheroptical disc drives.

These light emitting devices are made of nitride semiconductors. In thecase where the active layer thereof has a multiple quantum wellstructure, the piezoelectric effect due to lattice mismatch tilts theenergy band of the barrier layer and the well layer. In the tiltedenergy band, electrons are likely to be spatially distant from holes inthe well layer.

When carriers are injected into the multiple quantum well, it is likelythat there are more electrons than holes in the well layer located atone end of the multiple quantum well, and more holes than electrons inthe well layer located at the other end thereof.

Thus, the multiple quantum well made of nitride semiconductors has aproblem of decreased emission efficiency due to low carrierrecombination probability.

JP-A-2006-013463(Kokai) discloses a technique related to a nitridesemiconductor light emitting element which includes a light emittinglayer having a multiple quantum well structure with relaxed compressivestrain. In this technique, each of the barrier layers, which sandwichthe light emitting layer on both sides, contains impurity throughout thelayer, and the impurity concentration in the center portion along thethickness direction is higher than in the portion in contact with thewell layer. It is stated that this enables high emission intensity.

However, even in this technique, the energy band bending throughout themultiple quantum well layers constituting the active layer is small, andinsufficient for transferring carriers so that they are efficientlyconfined in the multiple quantum well layers. In particular, holes,which have a large mass, are more difficult to migrate than electrons.Hence, it is not easy to enhance emission efficiency.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a lightemitting device including: a first layer made of a semiconductor of afirst conductivity type; a second layer made of a semiconductor of asecond conductivity type; an active layer including a multiple quantumwell provided between the first layer and the second layer, impurityconcentration of the first conductivity type in each barrier layer ofthe multiple quantum well having a generally flat distribution orincreasing toward the second layer, average of the impurityconcentration in the barrier layer on the second layer side as viewedfrom each well layer of the multiple quantum well being equal to orgreater than average of the impurity concentration in the barrier layeron the first layer side, and average of the impurity concentration inthe barrier layer nearest to the second layer being higher than averageof the impurity concentration in the barrier layer nearest to the firstlayer.

According to another aspect of the invention, there is provided a lightemitting device including: a first layer made of a semiconductor of afirst conductivity type; a second layer made of a semiconductor of asecond conductivity type; an active layer including a multiple quantumwell provided between the first layer and the second layer, impurityconcentration of the first conductivity type in each barrier layer ofthe multiple quantum well having a generally flat distribution, theimpurity concentration in the barrier layer on the second layer side asviewed from each well layer of the multiple quantum well being greaterthan the impurity concentration in the barrier layer on the first layerside.

According to another aspect of the invention, there is provided a lightemitting device including: a first layer made of a semiconductor of afirst conductivity type; a second layer made of a semiconductor of asecond conductivity type; an active layer including a multiple quantumwell provided between the first layer and the second layer, impurityconcentration of the first conductivity type in each barrier layer ofthe multiple quantum well increasing toward the second layer, theimpurity concentration in the barrier layer on the second layer side asviewed from each well layer of the multiple quantum well being greaterthan the impurity concentration in the barrier layer on the first layerside.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a nitride semiconductor light emitting deviceaccording to this embodiment;

FIGS. 2A and 2B illustrate energy band bending;

FIGS. 3A to 3C illustrate the band of a nitride semiconductor lightemitting device according to a comparative example;

FIG. 4 is a graph showing characteristics for optical output andinternal (emission) efficiency;

FIGS. 5A and 5B show a variation of the donor concentration distributionin the barrier layer; and

FIG. 6 shows a variation of the impurity concentration distribution inthe barrier layer.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention will now be described with reference tothe drawings.

FIG. 1 shows a nitride semiconductor light emitting device according tothe embodiment of the invention. More specifically, FIG. 1A is an energyband diagram in the equilibrium state thereof, and FIG. 1B is aschematic cross-sectional view.

The embodiment shown in this figure illustrates a light emitting deviceused for a white LED light source and being capable of emitting light inthe ultraviolet to green wavelength range.

An active layer 20 of the light emitting device includes a well layer 20a made of In_(x)Ga_(1-x)N (where 0.05≦x≦1) and a barrier layer 20 b madeof In_(y)Ga_(1-y)N (where 0≦y≦1 and x>y). A first layer 11 of n-type ismade of Al_(t)Ga_(1-t)N (where 0≦t≦1.0), and a second layer 21 of p-typeis made of Al_(u)Ga_(1-u)N (where 0≦u≦1.0). More preferably, thealuminum composition ratios t and u are equal to or greater than 0 andequal to or less than 0.2.

A more detailed structure is shown in FIG. 1B. The first layer (n-typelayer) 11, the active layer 20, the second layer (p-type layer) 21, anda p-side electrode 26 are provided on a sapphire substrate 10. The firstlayer 11 includes a foundation layer 12 made of an n-type GaN layer, anda superlattice layer 14 made of n-typeIn_(0.1)Ga_(0.9)N/In_(0.05)Ga_(0.95)N for facilitating current diffusionin the foundation layer 12.

The active layer 20 illustratively has a multiple quantum well structuremade of In_(0.2)Ga_(0.8)N/In_(0.05)Ga_(0.95)N. The second layer 21includes a cladding layer 22 made of p-type Al_(0.2)Ga_(0.8)N and acontact layer 24 made of p-type GaN.

In the crystal growth of such nitride semiconductors, use of MOCVD(metal organic chemical vapor deposition) or MBE (molecular beamepitaxy) can increase production scale while maintaining goodcrystallinity.

The term “nitride semiconductor” used herein refers to semiconductorsrepresented by (Al_(x)B_(1-x))_(y)Ga_(z)In_(1-y-z)N (0≦x≦1, 0<y<1,0<z<1, y+z≦1), and further includes those containing any group V elementsuch as As and P, and those containing a p-type or an n-type impurity.

An n-side electrode 16 is selectively formed on the foundation layer 12,and a p-side electrode 26 is selectively formed on the contact layer 24.Light from the active layer 20 is emitted upward, laterally, anddownward. If a reflecting layer is formed below the sapphire substrate10, the light directed downward can be reflected and extracted upward orlaterally, which facilitates increasing light extraction efficiency.

In the energy band diagram of FIG. 1A, the horizontal axis X representsthe relative position along the perpendicular direction of FIG. 1B, withthe direction from the first layer 11 to the second layer 21 assumedpositive. Electrons in the conduction band 40 constitute an electroncurrent 46 toward the second layer 21 and are injected into the activelayer 20. Holes in the valence band 42 constitute a hole current 48toward the first layer 11 and are injected into the active layer 20.Here, the quasi-Fermi level is labeled with reference numeral 44. Inthis figure, the number of wells is eight.

FIG. 2 illustrates energy band bending. More specifically, FIG. 2A is anenergy band diagram in the vicinity of the active layer of FIG. 1A, andFIG. 2B shows impurity concentration in the barrier layer.

A nitride semiconductor is a material with large piezoelectric effect.Lattice mismatch causes a compressive strain in the plane of the welllayer 20 a and produces an internal electric field along the X axis inthe active layer 20. This internal electric field tilts the energy bandof the well layer 20 a and the barrier layer 20 b.

A width of the well layer 20 a can be illustratively in the range from 2to 8 nm. A width of the barrier layer 20 b can be illustratively in therange from 5 to 25 nm, and more preferably in the range from 5 to 15 nm.If the width of the barrier layer 20 b is narrower, it is difficult toconfine carriers. If the width of the barrier layer 20 b is thicker, thequantum effect is suppressed, and migration of carriers is madedifficult.

In this embodiment, impurity concentration in the barrier layer 20 b isgradually varied.

More specifically, the impurity concentration of the first conductivitytype in each barrier layer 20 b of the multiple quantum well has agenerally flat distribution or increases toward the second layer 21. Theaverage of the impurity concentration in the barrier layer 20 b on thesecond layer 21 side as viewed from each well layer 20 a of the multiplequantum well is equal to or greater than the average of the impurityconcentration in the barrier layer 20 b on the first layer 11 side.Furthermore, the average of the impurity concentration in the barrierlayer 20 b nearest to the second layer 21 is higher than the average ofthe impurity concentration in the barrier layer 20 b nearest to thefirst layer 11.

In the example shown in FIG. 2, the barrier layers 20 b are of n-type,and the donor concentration therein is gradually increased toward thefirst layer 11 (p-type layer). More specifically, the donorconcentration is determined so that the following formula holds betweenthe average donor concentration N_(j) in the j-th barrier layer B_(j)and the average donor concentration N_(j+1) in the (j+1)-th barrierlayer B_(j+1):

N _(j) <N _(j+1)

where 1≦j≦M (M being the number of wells).

In FIG. 2B, the donor concentration is constant (i.e., equal to theaverage) in each barrier layer 20 b.

For example, in the case where the first layer 11 has a donorconcentration of 1×10¹⁸ cm⁻³ and the second layer 21 has an acceptorconcentration of 2×10¹⁹ cm⁻³, the average donor concentration N_(j) inthe barrier layer B_(j) is gradually increased toward the second layer21 relative to the donor concentration in the first layer 11. Then, inthe active layer 20 made of a multiple quantum well, the entire energyband of the conduction band 40 and the valence band 42 indicated by thedotted lines can be gradually bent, which facilitates distributinginjected carriers into the respective well layers W_(j) and efficientlyconfining them.

In particular, this band bending allows holes, which have a large massand are difficult to transfer, to migrate more easily toward the firstlayer 11 (n-type). This facilitates efficiently confining them in therespective well layers W_(j), and hence enhancing recombinationefficiency in the multiple quantum well structure.

By way of comparison, the solid lines indicate a conduction band 140 anda valence band 142 in the case where the average donor concentrationN_(j) in the barrier layer B_(j) is generally the same independent ofits position.

In FIG. 2A, the slope of the (j+1)-th barrier layer B_(j+1) is moregradual than the slope of the j-th barrier layer B_(j). That is, thebarrier becomes lower as it comes close to the second layer 21, whichfurther facilitates migration of carriers.

While the first layer 11 and each barrier layer B_(j) are of n-type andthe second layer 21 is of p-type in FIG. 2, the conductivity type may bereversed. That is, the first layer may be of p-type, the second layermay be of n-type, the barrier layer 20 b may be of p-type, and theacceptor concentration in the barrier layer B_(j) may be graduallyincreased toward the second layer 21.

FIG. 3 illustrates the band of a nitride semiconductor light emittingdevice according to a comparative example. More specifically, FIG. 3A isa band diagram of the entire chip, and FIGS. 3B and 3C are band diagramsof the active layer.

In FIG. 3B, in a light emitting element (LED) with a large number ofwells, the barrier layers 120 b are doped with an n-type impurity (Si).The donor concentration is constant at generally 3×10¹⁸ cm⁻³ in eachbarrier layer 120 b, and generally the same independent of the positionof the barrier layers 120 b in the multiple quantum well 120.

In this case, because the well layer 120 a and the barrier layer 120 bare tilted, electrons 150 and holes 152 are likely to be spatiallyseparated in each well layer 120 a, and carrier transfer between thewell layers 120 a is difficult. In the active layer 120 as a whole, theconduction band 140 and the valence band 142 have no band bending. Moreelectrons 150 are accumulated in the well layer 120 a on the first layer111 side, whereas more holes 152 are accumulated in the well layer 120 aon the second layer 121 side. Thus, carriers are likely to be separatedto the well layers 120 a at both ends of the active layer 120.

Therefore, carriers are distributed nonuniformly in the active layer120, decreasing recombination efficiency. In particular, in the casewhere the amount of injected carriers is small, the recombinationefficiency greatly decreases, which results in significant decrease inemission efficiency. That is, in the comparative example of FIG. 3B,improvement in recombination efficiency is limited.

If the donor concentration in the center portion of the barrier layer120 b is made higher the donor concentration in the portion in contactwith the well layer 120 a, as indicated by the dotted lines in FIG. 3C,the barrier at the center of the barrier layer 120 b is decreased towardthe second layer 121, and the slope of the band at the interface betweenthe barrier layer 120 b and the well layer 120 a is decreased. However,this does not result in a band bending large enough to facilitatemigration of carriers.

In contrast, in this embodiment shown in FIG. 2A, band bending occursthroughout the multiple quantum well layers. Hence, carriers easilymigrate in the active layer 20, and electrons 50 and holes 52 can beefficiently confined in the M well layers W_(j). Thus, recombination isfurther enhanced in each well layer W_(j), which facilitates improvingemission efficiency while preventing carrier overflow.

FIG. 4 is a graph showing a simulation result for optical output andinternal (emission) efficiency. The vertical axis represents opticaloutput Po (W/m) and internal efficiency η_(in), and the horizontal axisrepresents current (A/m). Here, “m” refers to meter, representing theunit length in the depth direction of the cross section of FIG. 1B. Thesolid lines indicate this embodiment, and the dashed lines indicate thecomparative example.

For example, at a current of 250 A/m, the optical output Po of thisembodiment is generally 530 W/m, which is as high as approximately 1.12times the optical output Po of the comparative example, generally 475W/m. At the same current 250 A/m, the internal efficiency η_(in) of thisembodiment is generally 51%, which is as high as approximately 1.11times the internal efficiency η_(in) of the comparative example,generally 46%. Furthermore, at a low current of 50 A/m, the opticaloutput Po of this embodiment is generally 110 W/m, which is as high as1.29 times the optical output Po of the comparative example, generally85 W/m. At the same current 50 A/m, the internal efficiency η_(in) ofthis embodiment is generally 52%, which is as high as approximately 1.21times the internal efficiency η_(in) of the comparative example,generally 43%. Thus, this embodiment readily maintains high opticaloutput Po and internal efficiency η_(in) in a wide current range.

The number of wells and the impurity concentration distribution in theactive layer 20 are to be in a suitable range. For a nitridesemiconductor, because of its large stress/strain, it is typicallypreferable that the number of wells is 15 or less, and for a lightemitting element (LED), it is more preferably in the range from 6 to 12.In the case where the band is bent by varying donor concentration in thebarrier layer B_(j), an excessively large band bending results inincreasing the number of carriers that can be confined in the well layer20 a, and increasing the carriers passing through the active layer 20.This decreases carriers contributing to recombination, and may fail tosufficiently improve emission efficiency. Improvement in emissionefficiency is facilitated when the number of well layers W_(j) is 6 to12 and the average donor concentration is in the range from 1×10¹⁸ to2×10¹⁹ cm⁻³.

FIG. 5 shows a variation of the donor concentration distribution in thebarrier layer. More specifically, FIG. 5A shows the increase of theimpurity concentration distribution, and FIG. 5B illustrates the averageof the impurity concentration distribution.

In the case where the impurity concentration is not flat, the averageimpurity concentration Nav determined by integrating impurityconcentration n(x) in each barrier layer 20 b is given by formula (1):

$\begin{matrix}{N_{AV} = \frac{\int_{x\; 1}^{x\; 2}{{n(x)}\ {x}}}{{x\; 2} - {x\; 1}}} & (1)\end{matrix}$

where N_(AV) is average impurity concentration, n(x) is impurityconcentration, and x1 and x2 are end positions of the barrier layer.

In FIG. 2B, the donor concentration in the barrier layer 20 b is flat ata constant value. However, the invention is not limited thereto. Thedonor concentration may increase toward the second layer 21 in eachbarrier layer B_(j) (1≦j≦M). In FIG. 5A, the donor concentration N_(j)linearly increases in the barrier layer B_(j), but may increase in acurved manner.

Also in this case, the j-th average Nav_(j) and the (j+1)-th averageNav_(j+1) of the impurity concentration are determined so as to satisfyformula (2):

Nav _(j) <Nav _(j+1)   (2)

where 1≦j≦M.

Thus, if the impurity concentration is gradually increased in eachbarrier layer B_(j), the band bending throughout the active layer 20 isreadily controlled, which facilitates carrier transfer.

FIG. 6 shows a variation of the impurity concentration distribution inthe barrier layer.

The impurity concentration distribution in each barrier layer B_(j) isassumed to be flat. If it is not flat, it may be replaced by the averageimpurity concentration N_(AV) given by formula (1).

The requirement is that the average Nav_(M+1) for the barrier layerB_(M+1) nearest to the second layer 21 is higher than the average Nav₁for the barrier layer B₁ nearest to the first layer 11. That is, bandbending occurs and can facilitate carrier transfer as long as there isat least one region satisfying Nav_(j)<Nav_(j+1).

In FIG. 6, for example, the average Nav₂ for the barrier layer B₂ isequal to the average Nav₃ for the barrier layer B₃, and the averageNav_(M−1) for the barrier layer B_(M−1) is equal to the average Nav_(M)for the barrier layer B_(M). Also in this case, the average Nav_(M+1)for the barrier layer B_(M+1) nearest to the second layer 21 can be madehigher than the average Nav₁ for the barrier layer B₁ nearest to thefirst layer 11. Hence, band bending occurs and can facilitate carriertransfer. The crystal growth process is simpler for the impurityconcentration distribution shown in FIG. 6 than for the impurityconcentration distribution shown in FIG. 2B.

For a multiple quantum well with the width of the well layer 20 a beingin the range from 2 to 8 nm and the width of the barrier layer 20 bbeing 5 to 15 nm, for example, the multiple quantum well structure madeof In_(0.2)Ga_(0.8)N (well layer)/In_(0.05)Ga_(0.95)N (barrier layer)can be sequentially crystal-grown illustratively by MOCVD. In the casewhere the barrier layer 20 b is of n-type, Si produced by decomposingSiH₄ (silane) gas, for example, can be used as a donor.

For example, by using an MOCVD apparatus to automatically control theflow rate, reaction time, reaction temperature and the like of SiH₄ gasserving as a doping gas, gradual increase of donor concentration asshown in FIGS. 2B, 5A, and 6 can be readily achieved. In thisembodiment, the average impurity concentration Nav in the barrier layerB_(j) is gradually increased or decreased. Hence, the time required forcrystal growth of the multiple quantum well can be made generally thesame as in the case where the concentration is constant in the barrierlayer B_(j). Thus, the productivity of the crystal growth process isreadily enhanced, and this embodiment can provide a light emittingdevice with high manufacturability.

While the above embodiment provides a light emitting device based onnitride semiconductors, the invention is not limited thereto. It is alsopossible to provide a light emitting device made of InAlGaP-basedmaterials with small piezoelectric effect, which can emit light in thevisible wavelength range. For InAlGaP-based materials with smallstress/strain, the number of wells is illustratively 40. Also in thiscase, if the concentration in the barrier layer is gradually varied tobend the band, recombination in the multiple quantum well is enhancedwhile preventing carrier overflow, and emission efficiency can beimproved. Here, the InAlGaP-based material refers to a materialrepresented by composition formula In_(x)(Ga_(y)Al_(1-y))_(1-x)P (where0≦x≦1, 0≦y≦1), and also includes those doped with a p-type or an n-typeimpurity.

The invention is not limited to LED, but is also applicable tosemiconductor laser devices, in which the impurity concentration in thebarrier layer is gradually varied to bend the band so that carrierrecombination is enhanced and emission efficiency is improved.

The embodiment of the invention has been described with reference to thedrawings. However, the invention is not limited to the above embodiment.For example, those skilled in the art can variously modify the material,size, shape, layout and the like of the active layer, multiple quantumwell, well layer, barrier layer, cladding layer, contact layer, currentdiffusion layer, foundation layer, and substrate constituting the lightemitting device, and such modifications are also encompassed within thescope of the invention unless they depart from the spirit of theinvention.

1. A light emitting device comprising: a first layer made of asemiconductor of a first conductivity type; a second layer made of asemiconductor of a second conductivity type; an active layer including amultiple quantum well provided between the first layer and the secondlayer, impurity concentration of the first conductivity type in eachbarrier layer of the multiple quantum well having a generally flatdistribution or increasing toward the second layer, average of theimpurity concentration in the barrier layer on the second layer side asviewed from each well layer of the multiple quantum well being equal toor greater than average of the impurity concentration in the barrierlayer on the first layer side, and average of the impurity concentrationin the barrier layer nearest to the second layer being higher thanaverage of the impurity concentration in the barrier layer nearest tothe first layer.
 2. The device according to claim 1, wherein the activelayer includes the well layer made of In_(x)Ga_(1-x)N (where 0.05≦x≦1)and the barrier layer made of In_(y)Ga_(1-y)N (where 0≦y≦1 and x>y), thefirst layer is made of Al_(t)Ga_(1-t)N (where 0≦t≦1.0), and the secondlayer is made of Al_(u)Ga_(1-u)N (where 0≦u≦1.0).
 3. The deviceaccording to claim 1, wherein the first conductivity type is n-type. 4.The device according to claim 3, wherein an impurity concentration ofthe each barrier layer is equal to or greater than 1×10¹⁸ cm⁻³ and equalto or less than 2×10¹⁹ cm⁻³.
 5. The device according to claim 1, whereinthe impurity concentration in the each barrier layer is equal to orgreater than the impurity concentration in the first layer.
 6. Thedevice according to claim 1, wherein a width of the barrier layer isequal to or greater than 5 nm and equal to or less than 25 nm.
 7. Thedevice according to claim 1, wherein a width of the well layer is equalto or more than 2 nm and equal to or less than 8 nm.
 8. The deviceaccording to claim 1, wherein a number of the well layer is equal to orless than
 15. 9. A light emitting device comprising: a first layer madeof a semiconductor of a first conductivity type; a second layer made ofa semiconductor of a second conductivity type; an active layer includinga multiple quantum well provided between the first layer and the secondlayer, impurity concentration of the first conductivity type in eachbarrier layer of the multiple quantum well having a generally flatdistribution, the impurity concentration in the barrier layer on thesecond layer side as viewed from each well layer of the multiple quantumwell being greater than the impurity concentration in the barrier layeron the first layer side.
 10. The device according to claim 9, whereinthe active layer includes the well layer made of In_(x)Ga_(1-x)N (where0.05≦x≦1) and the barrier layer made of In_(y)Ga_(1-y)N (where 0≦y≦1 andx>y), the first layer is made of Al_(t)Ga_(1-t)N (where 0≦t≦1.0), andthe second layer is made of Al_(u)Ga_(1-u)N (where 0≦u≦1.0).
 11. Thedevice according to claim 9, wherein the first conductivity type isn-type.
 12. The device according to claim 11, wherein an impurityconcentration of the each barrier layer is equal to or greater than1×10¹⁸ cm⁻³ and equal to or less than 2×10¹⁹ cm⁻³.
 13. The deviceaccording to claim 9, wherein the impurity concentration in the barrierlayer is equal to or greater than the impurity concentration in thefirst layer.
 14. A light emitting device comprising: a first layer madeof a semiconductor of a first conductivity type; a second layer made ofa semiconductor of a second conductivity type; an active layer includinga multiple quantum well provided between the first layer and the secondlayer, impurity concentration of the first conductivity type in eachbarrier layer of the multiple quantum well increasing toward the secondlayer, the impurity concentration in the barrier layer on the secondlayer side as viewed from each well layer of the multiple quantum wellbeing greater than the impurity concentration in the barrier layer onthe first layer side.
 15. The device according to claim 14, wherein theactive layer includes the well layer made of In_(x)Ga_(1-x)N (where0.05≦x≦1) and the barrier layer made of In_(y)Ga_(1-y)N (where 0≦y≦1 andx>y), the first layer is made of Al_(t)Ga_(1-t)N (where 0≦t≦1.0), andthe second layer is made of Al_(u)Ga_(1-u)N (where 0≦u≦1.0).
 16. Thedevice according to claim 14, wherein the first conductivity type isn-type.
 17. The device according to claim 16, wherein an impurityconcentration of the each barrier layer is equal to or greater than1×10¹⁸ cm⁻³ and equal to or less than 2×10¹⁹ cm⁻³.
 18. The deviceaccording to claim 14, wherein the impurity concentration in the eachbarrier layer is equal to or greater than the impurity concentration inthe first layer.
 19. The device according to claim 1, wherein theimpurity concentration of the first conductivity type in the eachbarrier layer of the multiple quantum well has a generally flatdistribution, the impurity concentration in the barrier layer on thesecond layer side as viewed from the each well layer of the multiplequantum well is equal to or greater than the impurity concentration inthe barrier layer on the first layer side, and the impurityconcentration in the barrier layer nearest to the second layer is higherthan the impurity concentration in the barrier layer nearest to thefirst layer.
 20. The device according to claim 1, wherein the impurityconcentration of the first conductivity type in the each barrier layerof the multiple quantum well increases toward the second layer, averageof the impurity concentration in the barrier layer on the second layerside as viewed from the each well layer of the multiple quantum well isequal to or greater than average of the impurity concentration in thebarrier layer on the first layer side, and average of the impurityconcentration in the barrier layer nearest to the second layer is higherthan average of the impurity concentration in the barrier layer nearestto the first layer.